WO patent publication 99/14113 discloses a propulsion system for vessels and a method for moving a vessel in ice conditions. The system comprises a drive shaft, a propeller attached to the drive shaft, and a nozzle surrounding the propeller. The nozzle has a water inlet and a water outlet, and rotatable blades or vanes attached to a portion of the drive shaft which projects outside the water inlet for breaking and or crushing ice before the ice enters the nozzle. The point of maximum diameter of the blades or vanes is positioned at an axial distance from the plane of the water inlet which is 0.02 to 0.25 times the diameter of the propeller. The diameter of the rotatable blades or vanes is 0.6 to 0.8 times the diameter of the propeller.
U.S. Pat. No. 2,714,866 discloses a device for propelling a ship. The motor casing is in the embodiment shown in FIG. 4 attached to a vertical rudder shaft and can thereby be turned with the rudder shaft from the interior of the ship. An electric motor is positioned within the motor casing. A nozzle surrounding the casing is supported with flat joint pieces on the casing. The pulling propeller which is driven with the electric motor is positioned at the front end of the casing within the nozzle. The flat joint pieces are slightly bent so that they capture the helical motion of the water coming from the propeller. This causes the helical motion component of the resultant speed of the water stream to change to an axial direction and to be employed for shearing.
U.S. Pat. No. 8,435,089 discloses a marine engine assembly including a pod mountable under a ship's hull. The marine propulsion set comprises at least one pod that is mechanically connected to a support strut, a propeller that is situated at the aft end of the pod and that has at least two blades, and an arrangement of at least three flow-directing fins that are fastened to the pod. This arrangement of fins forms a ring that is substantially perpendicular to the longitudinal axis of the pod, said ring lying within a zone that is situated between a central portion of said support strut and the propeller. The propulsion set comprises further a nozzle that surrounds, at least in part, the propeller and said ring. Each of said blades presents an end with an edge coming flush with the inside wall of the nozzle so that the propeller constitutes the rotor of a screw pump. The fins are positioned before the propeller in the normal direction of travel of the ship. There are no fins after the propeller.
Nozzles are used e.g. in so called Dynamic Positioning (DP) vessels used in oil drilling. The nozzle forms a central duct with an axial flow path for water from a first end to a second end of the nozzle. The thrust produced by the propeller is amplified by the nozzle at low speeds. The nozzle may produce up to 40% of the total thrust at low speeds, whereby the propeller produces 60% of the total thrust. There are several propulsion units in such vessels and the vessel is kept steady in position by the propulsion units. A big thrust is thus needed at low speed in order to keep the vessel continuously in position in rough seas.